Magnetic motors can transform the energy of an external magnetic field into the kinetic energy of their motion. When moving collectively in a microchannel, the motors interact not only with each other but also with the channel walls, which leads to a wide variety of motion modes. In our work, the collective motion of magnetic Fe3O4 nanomotors in different fluids in a microchannel in a non-uniform magnetic field is studied. During the process of their motion, the nanomotors induce flow of the surrounding liquid and move together with the liquid convective flow. Due to the liquid deceleration at the channel wall, the velocity of the motors depends on the channel height. In the case of a magnetic field gradient increasing in the direction of the nanomotors' motion, the propagating front, separating the region occupied by the nanomotors, becomes unstable. By contrast, the diffusive motion of the nanomotors in the liquid prevents the development of perturbations on the front. Overall, the velocity of the nanomotors' motion and their spatial distribution were found to vary versus the channel height, magnetic field, concentration of nanomotors in the suspension, and the viscosity of the liquid.